A stretched film formed by stretching a resin is used in various types of display devices, as a variety of optical films each carrying out a desired optical function, by utilizing its optical anisotropy. For example, in the field of liquid crystal display devices, it is known to use the stretched film as an optical compensation film for optical compensation such as prevention of coloration and widening of a viewing angle, and use the stretched film as a retardation film additionally serving as a polarizing-plate protection film, by laminating the stretched film and a polarizer together.
Meanwhile, late years, great interest has been shown in a self-luminous display device such as an organic electroluminescence (EL) display device, as a new display device. A self-luminous display device has a potential to suppress electricity consumption, as compared to a liquid crystal display device in which a backlight is always in an ON state. Further, in a self-luminous display device in which a plurality of light sources, such as organic EL displays, corresponding to respective colors are turned on, individually, there is no need to provide a color filter which becomes a factor of deterioration in contrast, so that it becomes possible to further enhance contract.
In an organic EL display, with a view to enhancing light extraction efficiency, a reflector such as an aluminum plate is provided on the side of a back surface of the display. Thus, outside light entering into the display is reflected by the reflector, thereby causing deterioration in image contrast. In this regard, it is known to use a circularly polarizing plate laminated the above stretched film and a polarizer together on the side of a front surface of the display, as means for preventing the outside-light reflection to thereby enhance light-dark contrast. In some cases, such a circularly polarizing plate is also used in a so-called 3D liquid crystal display device capable of displaying a stereoscopic image.
The lamination for the above circularly polarizing plate needs to be performed under an arrangement in which an in-plane slow axis of the stretched film is inclined at a desired angle with respect to an absorption axis of the polarizer.
However, a commonly-used polarizer (polarizing film) is obtained through stretching performed at a high stretching ratio in a conveyance direction, so that the absorption axis of the polarizer is coincident with the conveyance direction. On the other hand, a conventional retardation film is produced through longitudinal or transverse stretching, so that the in-plane slow axis is oriented in a direction defining an angle of 0° or 90° with respect to a longitudinal direction of the film, in principle. Thus, in order to allow the absorption axis of the polarizer and the slow axis of the stretched film to be arranged in mutually inclined relation with a desired angle therebetween, there is no other way but to perform a batch operation of cutting a long polarizing film and/or a long stretched film into pieces at a specific angle, and laminating respective pieces of the polarizing and stretched films together, so that deterioration in productivity and lowering of product yield due to adhesion of chips are pointed out. Particularly, in a current situation where the organic EL display is being increased in size year by year, the technique of obliquely cutting an obtained stretched film and then laminating the resulting stretched film to a polarizer causes deterioration in film utilization efficiency, thereby deterioration in productivity. Thus, there is a need to improve the productivity.
In regard to this need, there have been proposed various long stretched film production methods designed to stretch a resin film in an oblique direction at a desired angle by using an oblique stretching apparatus, wherein the slow axis can be freely controlled to be oriented in a direction defining an angle which is neither 0° nor 90°, with respect to a widthwise direction of the film (see, for example, the following Patent Literature 1).
In such methods, using a so-called bending type oblique stretching apparatus, a long obliquely-stretched film having a slow axis in a direction defining a desired angle of greater than 0° to less than 90° with respect to a widthwise direction thereof is produced by: feeding a long film from a direction which is different from a film winding direction after stretching; conveying the long film while holding opposite lateral ends of the long film by a pair of grippers; and, when a direction of the conveyance is changed, causing respective moving distances of one of the grippers and the other gripper to become different from each other to thereby obliquely stretch the long film. The use of such a stretched film having a slow axis inclinedly oriented with respect to the widthwise direction makes it possible to produce a circularly polarizing plate by laminating a long polarizing film and the long obliquely-stretched film in a roll-to-roll manner, instead of the conventional batch-wise lamination, thereby drastically enhancing the productivity and significantly improving the product yield.
Further, even in a situation where the long stretched film is used in a large-screen display, the capability of producing a circularly polarizing plate by roll-to-roll lamination makes it possible to increase a utilizable area of the long stretched film, thereby significantly reducing production cost of the circularly polarizing plate.
However, when viewing an image in a black state of an organic EL display equipped with an circularly polarizing plate produced by using the above long obliquely-stretched film, a phenomenon that black is tinged with red or blue, and the hue varies depending on a position on the display, so-called “color unevenness”, was observed. It was also found that a level of the color unevenness (optical unevenness) varies depending on a region of the long obliquely-stretched film used when producing the organic EL display, i.e., a visual condition of the display varies depending on the used region.
As a result of studying the above background art, it was found out that, in a self-luminous display device in which a plurality of light sources such as organic EL displays, corresponding to respective colors are turned on, individually, very high contrast is achieved because there are few members such as a color filter which become a factor of deterioration in contrast, whereas color unevenness tends to be significantly observed because even slight variation in optical property causes color unevenness.
It was also found that, when the long obliquely-stretched film is wound into a roll shape and, after being subjected to long-term storage, laminated to a long optical film in a roll-to-roll manner to obtain a circularly polarizing plate, a state of color unevenness is different from that in a circularly polarizing plate obtained using the long obliquely-stretched film without the long-term storage. That is, it was found that the state of color unevenness in a circularly polarizing plate obtained using the long obliquely-stretched film becomes different depending on a length of the storage period of the roll-shaped long obliquely-stretched film.
As a result of further studying these background art, it was found that, in the long obliquely-stretched film produced using the above conventional bending type oblique stretching apparatus, winding displacement is more likely to occur when it wound into a roll shape after oblique stretching. It was considered that this is primarily because an orientation angle of elastic modulus is oriented in an oblique direction during oblique stretching of a long film, so that a winding tension is not uniformly imparted to the resulting long obliquely-stretched film in a widthwise direction thereof during winding.
Then, the inventor of this application corrected the winding displacement by adjusting a condition of tension during winding, and others. However, a distribution of strong and weak regions in terms of winding tightness was produced, resulting in occurrence of deformation (including sticking between contact regions of the film and formation of a concavity on a surface of the film) during long-term storage of the roll-shaped wound body. As a result, in an organic EL display equipped with a λ/4 plate obtained by laminating a polarizer and the long obliquely-stretched film after long-term storage together, the above color unevenness was not achieved.
The inventor also focused on a cyclicity of the orientation angle described in the following Patent Literature 2. However, the technique described in the Patent Literature 2 could not sufficiently improve the deformation of the roll-shaped wound body during long-term storage due to the distribution of strong and weak regions in terms of winding tightness.